Image registration is the process of aligning two or more images that represent the same object at different times, where the images may be taken from different viewpoints and/or with different sensors. Image registration is used in the medical imaging field to merge or compare the results of the same (monomodal) or different (multimodal) imaging modalities, such as magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), ultrasound (US) and computed tomography (CT). Common examples of image registration applications in the medical field are multi-modality registration of anatomical (e.g., CT, MRI) and functional (e.g., PET, SPECT) images for accurate localization of tumors as well as delineation of their respective shapes and sizes, as well as the registration of serial images taken at different points in time to monitor the progression or regression of a disease, or to do post-operative follow-up. Another example of image registration is brain atlas registration, in which a brain image is warped or compared to a predefined template to identify and label specific regions of the brain. In general, registration of images acquired using the same modality allows monitoring changes over time, while registration of images acquired using different modalities increases the structural and functional information about a certain organ or region of the body.
Automatic three dimensional (3D) image registration is a numerically intensive task and usually demands large execution times (e.g., on the order of minutes). Total execution time can easily exceed an hour, for example, when registering 3D cardiac image sequences (e.g., of 20-30 images).